Pletch,
As you are aware, or should be, the  official policy of the POC is TO PROMOTE THE CORRECT AND AUTHENTIC RESTORATION OF THE PLYMOUTH, etc, etc....

Ok.. that said, lets try to respond to your question....
There are a lot of things that you can do to improve the performance of your '41. starting with a complete removal and replacement of the complete power train, up grading to a late model Mopar unit or the unthinkable but common Chevy componets.
A lot of the guys have upgraded their engine to the later '58/59 engines which are 140 hp and a direct replacement for the original engine. The addition of an overdrive trans from a DeSoto/Chrysler or a later Plym, etc is also a nice touch. You can use Chrysler wheels which are 15" and wider, and will allow the use of your stock hub caps.
You can also add dressup accessories like dual carbs, a high compression head and headers/spit manifold. If you add these accessories you should keep in mind that your engine, or the engine you select will need internal improvements like an improved cam, better cooling and rebuilding/regearing the differental if you opt for the overdrive and/or want the car to cruise better on the highway.
Will you ruin your car if you up-grade the power, power train? If you do it correctly you will not ruin the car and/or lession it's value, in fact you will improve the resale value and desirability of the vehicle. It is a fact of life that well done "Restorods" sell faster and for higher prices than cars that are restored to stock..

My '39 conv has been very highly modified with a late model (2001) V8, AT, PS, PB, PW, AC, cruise, etc. Did I ruin a rare vehicle? I don't think so..... My car is safe at freeway speeds, it is realiable and a pleasure to drive.  I have said it before and will say it again...... Chrysler built great cars, my '39 being one of them, I improved on what they started and can't help but feel that Walter P. would approve of what I did....Bill

Pletch;
There are lots of things that you can do to improve the performance of your Plymouth. Suggest you post your question on the P-15, D-24 forum.

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Don Coatney

I have been driving my 41 special deluxe all summer and enjoying it but was wandering if thier are things that i can do to it to improve the proformance of the flat 6 (I like the car just not enough zip ) . I know thiers got to be a couple people in the same situation .